Pacific Rhododendron

Like all rhododendrons this native needs acidic soil, roughly pH 4.5 to 6, and shows chlorosis, thin growth and poor bloom in alkaline ground.

Yellow leaves with a distinct network of green veins, worst on the NEWEST growth at the shoot tips - that vein pattern is what separates it from a nitrogen shortage, which yellows the oldest leaves uniformly. Whole plants going progressively paler over a season rather than suddenly. Brown scorched leaf margins and shoot-tip dieback in advanced cases. Symptoms concentrated on the side of a plant nearest a foundation, wall, path, driveway, or a stretch of limed lawn. One plant affected while a neighbor that does not need acid soil looks perfect.

Correct the soil, and use iron only to buy time. Chelated iron sprayed on the leaves or drenched into the root zone greens a plant up within a week or two, but it wears off in a season and has to be repeated indefinitely - it fixes nothing. The real fix is lowering pH with elemental sulfur applied to the surface and watered in, retesting each season until you are in range, then holding it there with acidic mulch and acid fertilizer. Expect one to two years; there is no fast route. Note that other shortages look identical - manganese deficiency produces the same yellow-with-green-veins pattern on some plants - so a plant that does not green up from iron should be tested rather than dosed again. Where the pH is high because of nearby concrete or genuinely calcareous subsoil, the plant is in the wrong place, and moving it into a properly built acidic bed or container will succeed where years of amendment will not.

Test the soil pH before you plant, not after, and match the plant to the ground - an acid-loving plant in limy soil will never be truly healthy however it is fed. Where the pH is too high, acidify with elemental sulfur worked in the season BEFORE planting, since sulfur takes months of soil biology to work and cannot be rushed, and retest before you commit plants. On genuinely alkaline ground the honest answer is often a raised bed of acidic mix, or a container, rather than fighting the native soil forever. Keep an acidic organic mulch topped up - pine bark, pine straw, sawdust - and site acid-lovers away from concrete foundations and mortared walls, which leach lime for decades. Never apply lime, wood ash or mushroom compost where the roots reach, use acid-forming fertilizers such as ammonium sulfate, and improve drainage, because cold waterlogged soil hurts uptake even at the right pH.

Not an infection but a nutrient lockout caused by soil chemistry. Iron is abundant in almost every soil, but as pH rises it converts to forms roots cannot take up, so a plant that needs acid ground slowly starves for iron while standing in soil full of it. The symptom is unmistakable: new leaves go pale yellow or cream while the veins stay green, giving a fine green net over a yellow leaf. It shows first and worst on the newest growth, because iron does not move around inside a plant to be recycled. It does not spread between plants, does not respond to fungicides, and does not resolve on its own - left uncorrected the plant grows less each year, leaves get smaller and paler, shoot tips die back, crops fall away, and it eventually dies. Worst near concrete, mortar, paths and limed ground, and in cold, wet, compacted soil.

Neat crescent-shaped notches chewed into leaf margins, appearing overnight and building up through summer - unmistakable once you have seen it. Damage with no insect in sight during daylight; go out after dark with a flashlight and you will find the adults feeding. Wilting, dull foliage, or an unexplained slow decline in a plant whose leaves are also notched, which is the combination that points at larvae on the roots. Container plants that loosen in their mix or lift out with almost no fine roots left. Chewed, girdled bark at the root crown just below soil level on a young shrub that has collapsed.

The notching itself is cosmetic and needs no treatment on an established plant, and spraying for the adults is largely wasted effort since they feed at night and hide by day. The larvae are the target. Drench beneficial nematodes into moist soil in late summer or early autumn while the grubs are small and the soil is still warm - that is the single most effective home control and it works especially well in containers. Repeat the following year if damage continues. Hand-collect adults after dark from small collections of plants. Repot badly affected container plants into fresh mix, washing the roots and removing every grub you find, and cut your losses on a shrub whose root crown has already been girdled.

Check new plants before you buy, since weevils travel in nursery containers and that is how most gardens acquire them. Keep deep loose mulch and leaf litter pulled back from the base of vulnerable shrubs, because that is where the adults shelter through the day. Keep plants vigorous and steadily watered so root feeding is easier to shrug off. A sticky barrier band around the base of a container, or a smooth-sided pot, stops the flightless adults climbing back up. Encourage ground beetles, birds, and shrews by leaving some rough ground, and avoid broad-spectrum insecticides, which do nothing about the larvae underground and remove the predators that eat the adults.

Root weevils - black vine weevil and its relatives - do their obvious damage as adults and their serious damage underground. The adults are flightless, dull black or grey, about a quarter to a third of an inch long with a short snout; they feed only at night and hide in mulch and leaf litter by day, so most gardeners see notched leaves for weeks before ever seeing the insect. The larvae are small white legless grubs with tan heads that live in the soil, eat fine roots, and then strip the bark from the root crown itself, which is what actually kills plants. Males barely matter - the females reproduce without them, so one insect can start an infestation. Container plants and newly set shrubs suffer most, because a small root system has nothing to spare.